This invention relates generally to an exposure apparatus and a device manufacturing method using the same. More particularly, the invention is concerned with an exposure apparatus such as a stepper, for example, and a device manufacturing method using the same, suitably applicable to the manufacture of various devices such as ICs, LSIs, CCDs, liquid crystal panels or magnetic heads, by, for example, illuminating a circuit pattern of a reticle with pulse light from an excimer laser, for example, and by projecting the illuminated circuit pattern onto a wafer.
Recently, in an exposure method using an ultra-high pressure Hg lamp, various attempts have been made to enhance the resolution by changing the exposure wavelength to i-line from g-line. Also, many proposals have been made to use pulse light of a shorter wavelength, as represented by an excimer laser, to increase the resolution. The use of short wavelength light is effective to enlarge the depth of focus with the reduction in wavelength, to thereby improve the resolution.
The emission spectrum of an excimer laser, in a case of a KrF excimer laser, for example, is about 300 pm (full width at half maximum), and this is sufficiently small as compared with the full width wavelength of light from conventional ultra-high pressure Hg lamps. Taking the quality into account, the optical material usable in a reduction projection lens of an exposure apparatus which uses light in the deep ultraviolet region might be synthetic quartz or fluorite only. For this reason, taking chromatic aberration into account, even with a spectral band width of 300 pm, which is sufficiently small as compared with that of ultra-high pressure Hg lamps, the band width has to be reduced (band-narrowed) further by about two digits. Namely, the full width at half maximum has to be not greater than 3 pm (0.003 nm).
For a reduction of the spectral band width, generally, a band narrowing unit having a dispersion element such as an etalon or grating is used to band narrow a laser light to a spectral band width of about 1 pm. While the emission center wavelength of the thus band narrowed laser light may differ with the laser medium used, usually a wavelength which is in the vicinity of the emission gain by which a maximum output is obtainable is selected. For example, in a case of a KrF excimer laser, it is close to 248.35 nm, which is the center of the emission gain.
On the other hand, recent semiconductor device manufacturing apparatuses are required to provide a high resolution and yet a high throughput. Generally, the resolution depends on the numerical aperture (N.A.) of a projection lens system. Further, for a higher throughput, the size of a chip has become larger. This necessitates a projection lens system having a large N.A. and yet a wide field angle.
However, enlargement of the N.A. or the field angle of a projection lens system directly leads to a difficulty in lens design. Also, it results in a narrowed tolerance to production, making the lens manufacture more difficult. For the manufacture of a projection lens system, generally, a number of lenses are combined into a projection lens, and trial printing tests are carried out by using that projection lens system. Aberrations of the projection lens system are detected on the basis of the results of trial printing, and correction of the lens system is performed. However, this process needs the skill of an operator and takes much time. Thus, the throughput is low. Also, it is practically difficult to correct all products (projection lens systems) precisely and exactly.
As for a method of quantitatively measuring the performance of a projection lens system having a combination of a number of lenses, there is a method in which an interferometer is used to detect aberrations and, on the basis of the detection, the lens system is corrected. Interferometers use a laser as a light source. However, in a case where an excimer laser is used in an exposure apparatus as a light source, there would be no laser source which is suitably usable in an interferometer and which has the same wavelength as that of the excimer laser. An excimer laser of the type used in the exposure apparatus may be used as a light source for the interferometer. However, the coherency of excimer lasers is not high, and they are not suited to be used as a light source of an interferometer. Additionally, because excimer lasers provide pulsed light emission, stabilization of outputs is difficult to attain. Thus, with an interferometer using an excimer laser, it is difficult to obtain good precision in inspection of the performance of a high-quality projection lens system having a large N.A. and/or a large field angle.
Gas lasers such as a Hexe2x80x94Ne laser have good coherency and they provide continuous wave emission. Thus, gas lasers may suitably used as a light source of an interferometer. However, their wavelength differs considerably from that of excimer lasers. Thus, while taking chromatic aberration or film characteristics into account, it is difficult to use gas lasers for inspection of a projection lens system designed for use with the wavelength of an excimer laser.
In the vicinity of 248.35 nm, which is the center wavelength of an ordinary KrF excimer laser, there is a double harmonic wave (a wave having a wavelength a half of its original wavelength) of an argon ion laser, which can be produced by using a secondary harmonic wave producing element. However, such harmonics have a wavelength of about 100 pm, which is quite different from that of the KrF excimer laser. Thus, it cannot be used as a light source for an interferometer for inspection of a projection lens system designed for use with a band narrowed excimer laser.
It is accordingly an object of the present invention to provide an improved exposure apparatus.
It is another object of the present invention to provide an improved device manufacturing method.
In accordance with an aspect of the present invention, there is provided an exposure apparatus, comprising: irradiating means for projecting pulse light to a mask; and a projection optical system for projecting a pattern of the mask onto a substrate, wherein the pulse light has an adjusted wavelength such that it substantially coincides with a wavelength of laser light of a continuous wave.
In accordance with another aspect of the present invention, there is provided an exposure apparatus, comprising: an irradiation optical system for projecting pulse light of a wavelength 248.25 nm to a mask; and a projection optical system for projecting a pattern of the mask onto a substrate.
In accordance with a further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: providing a light source adapted to supply pulse light of a wavelength 248.25 nm; and projecting the pulse light to a mask so that a pattern of the mask is projected through a projection optical system to a substrate.
In accordance with a yet further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: providing a light source adapted to supply pulse light; and projecting the pulse light to a mask so that a pattern of the mask is projected through a projection optical system to a substrate, wherein the pulse light has an adjusted wavelength such that it substantially coincides with a wavelength of laser light of a continuous wave.
The projection optical system to be used in the present invention may include a lens assembly, a mirror assembly or a combination of a concave mirror and a lens assembly.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.